May 16, 2013 11:37 PM

It's oval shaped, reasonably regular, greyish in the most part, I guess weighs about 3lbs (1.367Kg). It looks just like a normal big pebble, yet there are a couple of really interesting features.

On the top there's a group of fossils, smail-like. However their lustre makes them seem almost metallic.

Similarly 2 thirds of the way down the side, there is a band of what appears like metallic crystals attached (not fused) to it. The band is brittle, and I have broken a small piece off. It has a crystalline structure, and from what I can see with a magnifying glass it looks silver in colour.

It's so metallic looking I tested with a magnet but it is not magnetic, though some of the colouration looks like rust.

It is from near Staithes in North Yorkshire.

I'm not really a fossil hunter but this has got me really curious. I'd love an explanation if you have one!

This is a concretion, partly nucleated on a small collection of ammonoids (probably ammonites, perhaps Pleuroceras).

Concretions often have concentring banding. There is indication of this in the bottom specimen, bottom-right: that curved layer attached to the surface. The brown colouration is indicative of iron, and some of the metallic-looking minerals you see are probably pyrites (a form of iron sulphide).

With concretions that have a nucleus, the nucleus is usually in the middle. Here that does not quite seem to be the case. It is not easy to give a confident explanation. The growth of concretions is sometimes influenced by the nature of the rock, and if a concretion begins to form near a contact between different types of rock, it may grow asymmetrically across them. Pehaps that happened here, but it is a weak hypothesis.

Thanks for such a detailed and swift response Mike. I see you're quite prolific!

Yes, they are bands of concentring metalic looking mineral that look like metal, the spooky thing being the ammenites look metallic aswell! Curiosity my make me chip at it and see if the ammenites appear metalic throughout but feels a shame to disrupt something so ancient.

As an alternative to chipping it, you could take it to a stonemason (ask your local crematorium where local gravestones come from) and ask them to saw it in half. That way, as well as seeing inside the fossils, you'd still be able to have it as a whole (albeit with a line down the middle), but you could also use the halves as little bookends or doorstops. You'd need to varnish the cut surfaces to 'see' them properly. Just a thought.